Frequently natural gas contains excess nitrogen, making it commercially unusable. If wellhead natural gas has more than about 10.0 vol % nitrogen, then it may not have a minimum heating value specified by a pipeline company. Until now, there has been no technology available to economically reduce nitrogen content in natural gas. As a result there are many capped wells that remain unused.
Pipeline natural gas can contain up to about 10.0 vol % nitrogen if C2+ and higher hydrocarbons are added to increase heating value to a nominal commercial standard heating of 1,000±20 Btu per standard cubic foot (SCF). The balance is predominantly methane, usually 80-95 vol %, and small amounts of carbon dioxide, usually 0.0-2.5 vol %.
In the past the nitrogen content posed no problem for the most common application of natural gas space heating, if the heating value was 1,000±20 Btu per SCF. There is a new use for natural gas as a fuel for fuel cells (e.g., ONSI Corporation's Phosphoric Acid Fuel Cell (PAFC)) wherein a nitrogen content over 6.0 vol. % can severely reduce useful operating life.
The PAFC typically has two main operating sections:                1. A steam reformer where natural gas is partially oxidized by steam over a catalyst to yield hydrogen and carbon dioxide.        2. A stack of bipolar fuel cells with concentrated phosphoric acid electrolyte wherein hydrogen reacts electrochemically with oxygen from air to produce electricity, heat and water.        
The reformer catalyst can promote a side reaction between nitrogen, if present in the natural gas, and hydrogen to form ammonia according to the following chemical equation: Although the conversion of nitrogen to ammonia is low, the upper allowable limit for useful stack life is 1.0 ppmv of ammonia in the reformed gas. The ammonia concentration appears to be directly proportional to the amount of nitrogen in the natural gas. It has been found that a PAFC reformer creates about 1.0 ppmv ammonia if the natural gas contains about 6.0 vol % nitrogen. The level of 1.0 ppmv ammonia is generally considered to be the maximum allowable ammonia content for optimum cell stack life. At 1.0 ppmv ammonia, a PAFC cell stack would have about six years useful operating life before it would have to be renewed or replaced.
In the cell stack, ammonia hydrolyzes to ammonium ion as ammonium hydroxide, reacting with the phosphate ion in the aqueous phosphoric acid electrolyte. The ammonium ion neutralizes the phosphate ion in an elementary acid base reaction, forming the salt ammonium meta-phosphate according to the following chemical equation:
 4NH4OH+4H3PO4→(NH4)4P4O12+8H2O  (2)
Converting a portion of the electrolyte to the neutral salt degrades its hydrogen ion conductivity and eventually the rated electric power output of the fuel cell. It has been found that a PAFC fueled by natural gas with an average 6.0 vol. % nitrogen content and the resultant average 1.0 ppmv ammonia created in the reformed gas has a useful operating life of about six years. It has furthermore been found that if the average nitrogen content increases to 8.5 vol. %, there will be a proportional increase in ammonia concentration and an exponential reduction in useful operating life to about 1.5 years.
The ratio of fuel cell operating life, and therefore degradation rates, appears to vary directly with the fourth power of the ratio of nitrogen concentration. The degradation ratio is the inverse of the life cycles ratio:(8.5 vol. %/6.0 vol. %)4=4.028; 1/4.028≈1.5 yrs/6.0 yr.  (3)(6.0 vol. %/8.5 vol. %)4=0.248; 1/0.248≈6.0 yrs/1.5 yr.  (4)
Elementary chemical reaction kinetics also supports this conclusion. An irreversible reactant rate expression for the depletion (−r) of the ammonium ion as ammonium hydroxide derived from the stoichiometric equation (2) is:−r=k×[CNH 4OH]4×[CH3PO4]4  (5)where −r is moles per unit volume depleted per unit time, k is the temperature dependent rate constant, and C is the reactant concentration in moles per unit volume. The reaction rate is fourth order with respect to ammonium concentration. In other words, the rate of ammonium conversion and hence the rate of electrolyte degradation varies as the fourth power of the ammonium concentration, which is directly proportional to the nitrogen concentration.